perhaps you should take a look at Lingua::EN::Sentence This will handle some common sentence splitting probs like acronyms/abreviations. Something like this may do all you need.

use Lingua::EN::Sentence qw(get_sentences);
open FH, "SomeFile.txt" or die "can't open: $!\n";
{
  local $/;
  my $all=<FH>;
}
my $sentences=get_sentences($all);  ## Get the sentences.
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Looking at your code, I don't think split will work like this (spliting an entire array in one). you will need to foreach through @all and split each element individualy or possibly play some fun and games with map.

my @all=<>;

print "all is:\n";
foreach (@all) {
    print "$_";
    foreach (split /([.,;])/, $_) {print "$_\n"}
}
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With the parenthesis round the split pattern you get the punctuation that split acted on stored in your array too.

From your later description I think you may want to look at splitting on something like [A..Z]\s*(\.|;) and as you are slurping in the entire file and not making use of the line breaks perhaps you want to try

local $/;
my $all=<>;
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then split $all into the array of lines Cheers,
R.